Identify the correct statement(s).
(A) In any lens, the optical centre is always at the centre of the lens.
(B) In a lens, if the curvature of its two surfaces is equal, the optical centre is at the centre of the lens.
(C) A light ray passing through the optical centre of a lens never shifts from the direction of the incident ray.
(D) A ray of light incident perpendicularly on a lens passes through its focus after refraction.
Choose the correct answer from the following given options.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (C) only
The optical centre is the point on the principal axis through which a ray passes without deviation; its position depends on the curvatures of the two surfaces.
(A) In any lens, the optical centre is always at the centre of the lens. — Incorrect. With unequal curvatures the optical centre shifts towards the more strongly curved surface.
(B) In a lens, if the curvature of its two surfaces is equal, the optical centre is at the centre of the lens. — Correct. For a symmetric lens the optical centre is at the geometrical centre.
(C) A light ray passing through the optical centre of a lens never shifts from the direction of the incident ray. — Correct. Near the optical centre the faces act like a thin parallel slab; for a thin lens the ray goes straight through undeviated.
(D) A ray of light incident perpendicularly on a lens passes through its focus after refraction. — Incorrect. This holds only for a convex lens: for a concave lens a ray parallel to the principal axis only appears to come from the focus, and a ray incident at the optical centre goes straight on.